This is the second model taken from the 28mm plastic '45mm Anti Tank Gun' set from the Plastic Soldier Company (PSC), the other can be seen (here) . This model depicts the 76mm infantry M1943 which used the same carriage.
There will eventually be a online video showing how this base was made detailing in particular the tree roots. This will be on Ade's own website (Wargaming for fun) and on YouTube (Wargaming For Fun).
Ade does a great job of editing his videos which actually takes a substantial amount of time. This video will have a new voice-over as the original involved a fair amount of industrial language as it doesn't show the bit where I managed to superglue my fingers to the acrylic base, or the important bit (i.e. making the actual tree roots) when I didn't realise the camera battery had run out.