I used bum tanks for a few year when racing the Condo 750’s and Yellow Mountain events, they work ok, used to run the bike on those to it misfired and switched the front tank on and the rear of. I got caught up on a bum tank once and had a bad accident since then I haven’t used them and used different size tanks I am lucky as I have a 10.5L, 17L and a 28/30L mother.
Chris there is a thread regarding long range tanks, I think if you want someone with experience with a larger tank on a WR250 you maybe stretching it as bigger tanks normally are for long miles and the WR250 wouldn’t normally be the bike of choice, you will have to pioneer the way!
5L bum tank would stretch you out to 200kms with the bike geared right and that would be heaps on a little 250 and that seat and like ECKS said you can take it of, only leaves two 6mm holes :ohmy: , two rubber grommets from Supercheap can fix that 
TB