The mast support is to rest the mast on when the mast is lowered, the mast could be lowered on the real craft to be able to get under low bridges or to be less obtrusive in low treed area's when the radar wasn't required.
Start by laminating 3 strips of 1/32" ply around a paint bottle lid glued together with superglue.
After sanding the surfaces of the arch glue the ply upright around the curve, here I used a leftover from the torpedo roll-off-rack as the radius was correct at 14mm. Then use some styrene "U" channel around the upright and make some small feet out of 1/16" ply.
Line up the support correctly before gluing in place on the rear of the day cabin roof.
Make sure the Mast Stay slides unobstructed under the arch and the mast sits neatly on the cross piece.