This ESP32 one is perhaps a more 'pure' example in terms of the user experience, and is similar to the PicoZX in some regards: https://www.tindie.com/products/bobricius/picozx-handheld-zx...
But some of the others come with extra bells and whistles:
Most recently, The Spectrum, which is not without its detractors: https://retrogames.biz/products/thespectrum/
Here's one you can take on the go: https://retroradionics.com/OMNI-128HQ-LAPTOP-p75106221
Of course the super Spectrum Next, which is scarcer than hens teeth: https://www.specnext.com/
But has its own clones: https://ultimatemister.com/product-category/zx-spectrum/zx-n... & https://zxrenew.co.uk/Xberry-Pi-p596973674
And here are some more DIY options: https://www.bytedelight.com/?product_cat=harlequin128 & https://retrocomp.cz/produkt?id=62