I ended up ordering 80mm round ones for my mortars/quad launchers. But an oval base could work just as well, the 90x52 ones would be a good fit I think. Edit: they won't fit on the 75×42, either the wheels or part of the carriage will hang over the edge of the base.
Nice! You might want to shop around for the bases a bit; I got mine online and even with the added shipping costs they were much cheaper than getting the GW ones at a local store. The (round) ones I ended up getting were €0,79 a piece vs the GW ones that are priced at €4,00 each.
The hobby is expensive enough as is, and I'd rather spend my cash on more minis than on little plastic discs for them to sit on.