I tried a 6 x 2 timber between jack and rear axle. The timber just bends and offers no support to the vulnerable axle twist beam. I screwed and glued a second 6 x 2 alongside the first to stiffen the thing. It still bends under load but is probably good enough.
HOWEVER - its such a pain in the arse to align and centre, there's zero point to using it. The wood from mine got used on a house DIY job.
Put the jack under the end of the swing arm and lift up fully. Put axle stand under sill (with wood protection) and repeat the other side. Side 2 is quicker, because its wheel is almost off the floor anyway. If you really want to go higher, put a spacer between jack and suspension swing arm.
It's a non - problem. The timber beam (or even a steel beam) offers no added benefit.