An inexpensive option is to add a third lap belt on the rear bench.
They can be purchased at any auto parts shop, and can be bolted down
by sharing the same bolts hold that existing belts.
Installation is largely blind, working through two holes in the rear
seat frame; you will need a long socket extension and a little piece
of tar in the socket to hold the bolts as they are mounted straight
down. If you are lucky (as I was) the tar will still be stuck to the
bolt head from original assembly (!) but you won't know until the
bolt that you are removing drops ...
