If you're just trying to beef up your constructs hit points without increasing its HD, something you may have overlooked (I believe this is in Ultimate Magic, based on the footer for the page on d20pfsrd, though I don't have a copy near me to check) is the Bioconstruct Modification. This increases the CR by +1 and costs 22,750 GP per organ, of which there are 2 listed:
- Heart: This upgrade permits a golem to receive half the benefits of magical healing and provides a hit point bonus as if the golem had a Constitution score of 12, but the construct does not gain a Constitution score in this process. Negative energy spells cast upon the golem can affect the heart, causing it to cease functioning for the duration of the effect (or 1 round for instantaneous effects). If the spell has a save, the heart uses the golem’s saving throws.
- Brain: This upgrade doesn’t permit the golem to think independently, but instead, the brain works as a storage device, permitting the golem to acquire skills and feats as if it had an Intelligence score of 10. Enchantment spells or spells with mind-affecting effects that are cast upon the golem can affect the brain, causing the brain to cease functioning for the duration of the spell. While the brain is suppressed in this fashion, the construct temporarily loses access to its skills and feats. If the spell has a save, the brain uses the golem’s saving throws.
By adding the heart, your hit points are determined as if you had a 12 Constitution (so a +1 modifier), though you don't actual gain the Constitution score. The brain allows you to take skills and feats as if you had a 10 Intelligence, which allows you to take Toughness.
Adding the Bioconstruct Modification, in addition to the weaknesses listed in each organ comes with an additional weakness:
All bioconstruct upgrades have the same weakness—they are susceptible to critical hits. An attacker that confirms a critical hit against a golem with a bioconstruct upgrade deals damage to the construct and also destroys one upgrade. The damaged upgrade ceases to function and the construct loses abilities associated with the upgrade. If a construct has more than one bioconstruct upgrade, only one is damaged. The GM randomly determines the damaged organ