Most of this is actually covered by the rules if you dig around.
Savage Worlds does model facing. It just doesn't make a big deal about it most of the time. The shield rules reference the in-fiction facing this way (SWD, p. 50):
If a character with a shield is hit by a ranged attack from the protected side, roll damage normally, but add the Armor bonus of the shield to the character’s Toughness (it acts as an obstacle).
The rules have a range of shields from bucklers to large shields (p. 53), with their parry bonus determined by their size. (Note that these are all wearable shields. There are no stats for the unwearable kind of tower-shield, which can't be used in melee anyway and so would give no Parry bonus, so are best represented by the obstacle rules on page 75.) The parry bonus of this crafted shield will depend on its size: either +2 for the largest size (largest that can be worn, that is) or +1 for the two smaller sizes.
Armour value versus missile weapons is also included in the stats for shields on the same page: +2 Armour against shots that hit for medium and up. (Bucklers grant no bonus, as they are not large enough to count as obstacles.) This is, again, only for shots that come from the protected angle.
There are no rules for shields extra special armour, but they can be inferred from other rules (which is the Savage Worlds way of doing things). If this is just a door of heavy plating, then once it's pared down to a carriable weight and size it might not be any more special than a normal shield and not need any extra stats to reflect the original vehicle's heavy armour. However, if this door had extra bullet-stopping technology beyond "put lots of metal in the way" that is light enough to use as part of the shield, then I'd look to the various anti-ballistic armours for how Savage Worlds models that. The Kevlar vests, in particular, simply have an extra effect: they remove a certain number of AP from the incoming projectile, which makes the armour's AV more effective. The regular Kevlar Vest, for example, negates 4 AP of incoming projectiles.
Weight is going to be up to you though. What seems sensible? That's what Savage Worlds wants you to stat this thing with: what seems sensible. Personally I'd just use the weights of the shield sizes in the book. Since those reflect wood with metal banding, that's probably about the same as modern composite armours.
So there you have it. These are stats I would use for this makeshift shield, depending on size:
Shield Armour Weight Cost Notes
HUMV Buckler — 8 — +1 Parry, negates 4 AP*
HUMV Medium Shield — 12 — +1 Parry, +2 Armour to ranged shots that hit, negates 4 AP*
HUMV Large Shield — 20 — +2 Parry, +2 Armour to ranged shots that hit, negates 4 AP*
* See Kevlar Vest description, SWD p. 50