Broken bed

Go to the local DIY store ( thats open ) and get them to cut a piece the size you want. they may even have an offcut thats close to size that they can trim to size for you. 

£25 will buy a jig saw and reduce faff. 

If half inch and likely to be sat on I would want plywood rather than chipboard over that span

OK, useless info. About 30 years ago I was staying with family on a CL in the North of England. The table made up the base of a double bed and rested on two strips of hardwood screwed and glued onto the face of the front bench seats. The glue on one side had dried out and whilst halfway through a morning cuppa whist sat up with my brew the strip towards the head end gave way. I went down, the cup went down and the tea seemed to remain in the air and then drop back into the cup. We drove later to the nearest small town. No wood available but bought some brass screws and adhesive. Rooted through the town and found a builder chap renovating a property and asked where I could buy wood. 'Nothing local but grab a pint and come back in half an hour'. 

Took family to local pub, ordered the food et and a pint for me. Went back to the builder and he said to jump in the van. He drove about 7 or 8 miles to where the local coffin maker was. He rooted through his stock and brought out a matching strip of hardwood. No charge. Builder drove me back stopping at his home and made us both cheese and onion sandwiches before driving back to his job. What do I owe I asked. No charge. Gave him 4 tins of beer a a thankyou. We went back to site and borrowed a hand brace and some bits and a saw and repaired the van. Don't you like nice folk. 

"Marine plywood is stronger and more durable than regular plywood, which has only three layers. ... As a result, you can use a thinner sheet that will have the strength of a thicker, regular sheet of plywood. In addition, it doesn't have as many voids as regular plywood, which makes it structurally stronger."

I was quoting from the industry itself and that bank of understanding contains many, many accumulated decades of experience and knowledge. Marine ply is inherently stronger by construction and useful in situations like that described by the OP.

That's right CY. Coming from a town with a fishing heritage 'proper' BS 1088 marine ply was easier to find maybe. But we could often pick up off cuts reasonably cheaply for jobs which called for quality and strength.

Yes, WPB is not marine ply, despite some caling it so. However it is better than the 'regular' ply that some caravan manufacturers use in their construction. If you can find the pukka stuff I would for a weight baring bed base.There again may be I'm the old school belt and braces type. I do like to see quality jobs being done even in those repair ones.

Well agree with you there mickysf, I too like to see a proper job done. However I am rubbish at D.I.Y and usually end up paying to have it done properly. I have the know how but not the ability.