Reading between the lines in my own text I think I was suggesting the 9.2" howitzer model is likely to be a semi scratch build though nothing really awkward. The earth box was camouflaged with vegetation or even sandbagged and most heavies had awnings to take camouflage nets. These howitzers were often sited in wooded areas or on a tree line for camouflage and covered with tarpaulins so again a basic shape could be used if the howitzer was not being fired. The shells were heavy about 290 lbs each so 4 soldiers would be less of a Hernia inducing lift. From pics and films it seems that the shells were rolled (like full beer kegs) and lifted in a cradle by 4 soldiers for the last bit. see http://www.criticalpast.com/video/65675048361_British-soldiers_Battle-of-Somme_firing-a-Howitzer_World-War-I
and same with a bit more detail https://www.youtube.com/watch?v=kDwRwLuZ_4k
From the film there seems to be about a dozen men round the gun when loading. Hatless and with Wellie boots some of them could almost be farm labourers. The shots of the shells bursting are of course from a forward trench.