a few mistakes u committed.
1. no meter to measure the humidity level, so how u know the current level is good for your equipments?
2. related to 1. W/o the meter, since you do not have the readings of the humidity level, chances are it might be too dry or not enough dry. So it defeats the purpose. And air tight means moisture is locked tightly, without proper humidifying, your equipment is more prone to fungus!
3. again related to 1. W/o the meter, even though you might have enough silica gel for the existing equipments (by chance) , did u take into consideration, next time when u add more equipments you had to adjust the amount of silica gels to the correct humidity level. And another thing to factor, what if your equipment comes into contact with water and you did not wipe dry enough to place back into the dry box, is the amount of silica gel sufficient to suck away the moisture?
4. You might be hardworking enough to ensure you change the silica gel often but you sure you are able to continue doing these for the next 5 years (using the warranty status of a dry cabinet as a gauge of timeline) and not sick of it? I doubt so, human are lazy by nature and after sometime you will slack and heck care for sure.
It may be cheap but not a feasible solution in the long run. So i wun risk it on my equipments. I rather pay $80-90 more to give me peace and mind for the next 5 years as well as not worrying if the humidity level is constant unless you are very unlucky, your dry cabinet breaks down w/o u knowing!