I think it's a fun idea, but needs some work-shopping, since equipment never provides you cover.
A basic shield weights 6lbs, now it weights 48lbs. Nothing to scoff at, but not that for off from tower shields, and if you have an 18STR character or something, it's not all that crazy. The character however isn't exactly proficient with a shield size (and weight) he never trained with (presumably). As a result it gives the same +2 to AC as a regular-sized shield, no higher, but he can use a bonus action to plant it in the ground to make one border of a square a wall, granting full cover from attacks that pass through that border when huddled in the square "behind" the shield. Alternatively, he can plant it sideways to create a two-square border that grants half-cover from attacks on the other side (so space in the cover zone for two people). The alternative option is more game-y since the shield doesn't stretch for 10-feet (two borders) of course. Bonus action to yank it out of the ground again, and he should be good to go.