Hi Littledave,
Unfortunately the 'Wespe' is based on the Panzer 2 chassis with a 105mm Howitzer....and the 'Marder' came in 3 variants.....
The Marder 1 used the hull from the captured French Lorraine tank , the Marder 2 used the Panzer 2 chassis and the Marder 3 used the excellent Czech 38t chassis.
With the Panzer 3 chassis you are a bit more limited, because it was a good little tank especially with the 50mm gun and then as the STuG 3 armed with the 75mm gun and the 105mm that I mentioned earlier.....AND according to Wikipedia...(which is not always to be trusted)....there was this version.....

> Sturm-Infanteriegeschütz 33B - A close-support, Assault gun. Armed with a 15 cm sIG 33, 24 built. <<<
150mm gun....now THAT is nice I will try and find a picture or two.
P.S. Andy no problem about the mesh scale....my mind is good at useless facts

20mm at 1 /16th scale is 1.25mm so I would have thought that a splatter-guard is probably close...and whos to say that
ALL the mesh was 20mm
Alb.