Hi, so I have this mould that will be used just one time and theb scrapped.
It is made with epoxy gelcoat and a few layers of 200gsm cloth laminated with epoxy.
I want to infuse the part and I think the mould is too flexible for the job.
What do you suggest as the cheapest way to stiffen it?
I was thinking about breather cloth impregnated with cheap polyester resin but maybe you have better and cheaper ideas?

Thanks for any advice!

Depending on the size & shape of your mold, it may be more effective to cut some strips of MDF (or whatever other stiff material you have around) roughly to match the outside (or back) of the mold, attach them to the mold with resin & glass. Any leftover fiberglass strips you've got laying around should work fine. 

Really, pretty much anything rigid that you can attach to the mold should work. I've got parallel strips of 1/2" MDF on the back of a large hood mold (5' x 4'). It was a lot cheaper than adding many layers of fiberglass to an already-expensive mold, and I was able to cut some handles into the strips so it is easy to move and hang out of the way on a wall. MDF has the benefit of being cheap, easily available and very dimensionally-stable.