If you do a search on the web under ‘hold down bracket for a car battery’, you will find a world of possibilities out their, with a pretty good choice for under £10.
WRT batteries; on the web go to Tayna Batteries, mail order, very competitive pricing and an amazing range of types & sizes. They list a range of batteries for different Morgan models. OR, do a generic search under say ‘classic car batteries’ and you find a search tool by physical size, terminal type, terminal orientation, combine that with a cardboard cut out of the actual space available on your car and get the highest power/ biggest one that will fit.
I’ve now got a retro fitted shock absorber hoop on my +8 and that forms my constraint on battery size.