I’ve been looking for exactly the same thing recently. Go to Amazon or eBay and look up 25mm Velcro strap (or 1’ Velcro strap if in America). The 30cm length ones are too short for my upper forearm, 50cm long is bordering on too long; I’ve tried both. I’ve just ordered some of these 40cm ones, so hopefully they’ll be just right https://www.amazon.co.uk/dp/B015NLSF..._62TPAb8559160
Adendum: I’m not bothered about the wrist HR when using the watch in this configuration, as you simply thread the strap over the bars and underneath the watch. However, if you get a longer Velcro strap you can configure the strap so that there is a double thickness of strap around your arm, leaving the underside of the watch in contact with your skin.
