Yes: don't. You've got a kit of an M2 halftrack, which didn't exist in a "British 'rounded ' variant"
Potted history of US halftracks: the M2 halftrack was designed as a general-purpose vehicle and the M3 as an APC (officially, they were the Half-track Car M2 and the Half-track Personnel Carrier M3). The M3's body is longer than that of the M2, has different interior arrangements and a rear door that the M2 lacks (a supposed M2
with a rear door is actually an 81-mm Mortar Carrier M4), while the M2 (and M4) have hatches in the sides that the M3 lacks. These were all made by Autocar, Diamond-T, White and others, but because there was a need for more halftracks than these could produce, International Harvester Corp. (IHC) was also asked to build them. However, IHC adapted the design to its own production capacities and preferences, which produced the M5 and M9 halftracks. The M5 is the equivalent of the M3, the M9 of the M2, and
these are the ones with rounded rear corners because the side and rear are made from a single plate of steel. There are many other differences between IHC halftracks and those of the other manufacturers, which means you can't just round off the rear corners on an M3 to make an M5.
What's more for your particular idea is that first of all, the M9 is a "long-body" halftrack like the M5, rather than being shorter like the M2 is relative to the M3. Furthermore, there were no "basic" M9s made at all, only M9A1s, with the ring-mounted machine gun over the front passenger seat. On the inside, the M9A1 is much like an M2, but on the outside, it is 100% indistinguishable from an M5A1: it has a rear door but no hatches in the sides, for example. If you can't see inside the rear area,
you cannot tell if a halftrack is an M5A1 or an M9A1.
The US Army standardised on the M2/M3 series (and their derivatives) for use in Europe, so as to avoid the logistics of supplying spare parts for the M5-series as well. They did, however, supply the M5-series and most of its derivatives (chiefly the M9A1 and the Multiple Gun Motor Carrier M14) to the British — and ended up using a few M5s itself as well in Europe, but not a lot. (The US did use the M5 for training in the USA, though.)
Also speaking against your idea is the fact the British didn't use any American halftracks in North Africa, to the best of my knowledge
They did in Italy and in North-West Europe, including M14s converted into APCs, command posts, ambulances, etc. as well as M5s, M5A1s and M9A1s.
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