Stronger and sturdier don't really go with portable, folding or low budget!
Plinth couches are the industry standard for continued use priced at around £800 +
They will last a life time and have a operation and parts gaurantee for life. I love mine and wouldn't part with for any other, very good product.
I don't think you will find a 'budget' couch that will be suitable for larger clients.
If you are on a budget i would suggest staying away from metal ones and getting a decent wood version. I also have a wooden portable couch that i bought when i first started training, its a Craft Master couch which is very good, but as all cheaper couches they will wear over time and that means creaks and noises.
Have you tried taking your couch apart and refitting all the screws tightly, sometimes this can give a couch new life.
Other wise try this one
Special Edition Massagetable - Massage Tables & Yoga Mats from Zenthings.com, Ireland, UK
The main difference with these couches is that the it has the frame fixing across the bottom of the frame, most couches are designed to fit your legs under so the strength is only at the top.
What kind of therapy do you provide?
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