Without spending an obscene amount of money for an oven is it possible to create one large enough for like a 16’ boat hull with high temps ranging 250-300 f?

If so what kind of things do I need? This area is not something I’m intimately familiar with.

I made my 6' x 6' x 3' using 2" x 2" timber frame, lined both sides with 3/8" ply infilled with an insulation of your choice that will survive the temps you plan to use. I am using a standard oven heating element and fan for heat air air circulation (but fitted a better blade) and an Omega programable PID for control.

What kind of temps are you achieving? And are they sustained?

Yeh something like Steve built is perfectly fine, composite curing ovens are really just glorified hot-boxes. If you needed to reach 250-300° Celcius then it would need a bit more, but ~120°C is not hard to achieve or maintain. You can spend as much or as little as you like. I've also got a timber oven, with the inside made from fibre-cement for it's lack of heat transfer.

My three pieces of advise would be:

1. Spend money on a good controller. The PID is the brains of the operation, you want it to be very very good. Don't get a cheap one off eBay, get a nice expensive one from Omega. Just make sure you get one with an internal power supply to switch the SSR! I overlooked this and have had to wire in a 9V battery to switch the SSR. Works just fine, but I've messed up two or three cooks where the battery has died and the PID hasn't been able to switch the element on...

2. For a big oven, I would build a metal frame. Even for my small oven I kind of wish I'd built a metal frame. It would have cost more and the oven would be a little less energy efficient, but the timber does suffer over time from the heat cycling. Mine is a good 4 years old now and still fine, but the doors don't close as nicely anymore and I feel like it's going to beed a rebuild soon. On a large oven, this will be all the more evident.

3. Even heating of the oven is absolutely critical. For a big oven, use 4 or 5 or possibly even more heating elements. You wont over-heat the oven if you have a good PID, but if you don't have enough heating elements to heat the space quickly then the PID will crank the element on full to try and achieve your ramp rate. This could mean one section of your oven hits 200° while another part is only at 80°. This will destroy moulds and components and is possibly even dangerous.

You said you used fiber cement boards. Like the ones I can buy from Home Depot for bathrooms and or siding?

Also why is SSR?

I agree with everybody here.  SSR and PID controllers are great.  Circulating fans are key.  Fire-proof insides are a good idea.  It's still dangerous and you really have to be careful that stuff can't get sucked into the heaters.  And 120C is pushing the upper end of what is comfortable for a home-made oven...  I don't really see the advantage of curing low-temp pre-pregs above 90-100C given how hard it can be on tooling, CTE issues and problems with foam cores.  There are lots of good resin systems that work beautifully in this range.

Here's a post on making and buying ovens:  https://explorecomposites.com/articles/tools-and-equipment/oven-options-for-pre-preg-curing/

But really, be careful!  I have had oven heater fires (and close calls) and they're scary!

I did simple 4x4cm metal frame, insulated with 5cm alu sided panels, heated by 2kw industrial blow heater and cover everything with 25mm wood boards.......still runing cheap ebay PID controler trought SSR as i was only using it for infusion post cures (60 deg).

Dimensions are 200x100x90

Next step will be Omega PID and perhaps one more heating/blowing element if needed (tho i plan to use low cure cycle for prepegs).