I'd let a warforged alters self into construct even if he does not have any level in Juggernaught. After all, he's a construct. And even when self-altered, he keeps his Living Construct subtype and the associated strengths and weaknesses.
Monster Manual v.3.5.: The construct list here features animated objects, golems, homonculus, inevitables, retriever, shield guardian.
Monsters of Faerûn: The construct list here features crawling claw, golems, helmed horror, tomb tapper.
Monster Manual 2: The construct list here features automatons, bogun, bronze serpent, captured one, chain golem, clockwork horrors, dread guard, golems, half-golems, juggernaut, nimblewright, raggamoffyns, rogue eidolon, runic guardian.
Fiend Folio: The construct list here features blackstone gigant, caryatid column, golems, invitables, iron cobra, maug, necrophidius, wicker man, zodar.
MM3: The construct list here features cadaver collector, golems, grisgol, shredstorm, slaughterstone constructs, warforged.
Keeping only those with 5 HD or less, and within one size category of Medium, we have.
Small Construct:
Electrum Clockwork Horror (4 HD, MM2)
Raggamoffyn, Tatterdemanimal (1 HD, MM2)
Small Animated Object (1 HD, MM)
Warforged Scout (no HD, MM3)
Medium Constructs:
Automaton, Hammerer (5 HD, MM2)
Automaton, Pulverizer (3 HD, MM2)
Dread Guard (5 HD, MM2)
Iron Cobra (2 HD, FF)
Medium Animated Object (2 HD, MM)
Necrophidius (2 HD, FF)
Raggamoffyn, Common (3 HD, MM2)
Large Constructs:
Large Animated Object (4 HD, MM)
Maug (2 HD, FF)
Warforged Charger (4 HD, MM3)
The animated objects can be interesting for mobility, since you get the movement speed of your altered form. By carefully chosing the object you imitate, you can gain very fast walk (well, wheeling) speed, swim speed, climb speed, and even (clumsy) flight speed!