I'm afraid locating the magazine is all you
Mark does do some very interesting scratch builds.
I take it your looking to do one the Samson cars. If you are then converting a toy car to the 1st Samson car ( top picture ) can be done in under an hour.
Or if your inclined to the second Samson desgin which sore more action then quite frankly you'd be better off leaving the toy cars alone.
It's worth noting that not all early Armoured cars were Rolls Royce,( especially the ones your most likely thinking of and that includes Those with The 1914 pattern body.) But it's a bit rivet counting when it comes to 28mm. Talbots were also used,as were a few others( only one of the two lower photographs is a Roll's but it just shows for gaming it doesn't matter that much.)