I rolled my own when I needed a similar function (it was very handy when adding some interactive components to a WordPress site).

Here's the code I used:

/**
 * Turn a svelte element into a simple web component
 * The content inside the web component will be passed in as the `slot` attribute and it's of type: `Array<ChildNode>`
 * @param svelteElem 
 * @param elemName 
 */
export function elementify(svelteElem: any, elemName: string) {
  class newElem extends HTMLElement {

    svelteComp: any

    constructor() {
      super()
      //at this point the internal elements are not mounted yet, so we need to do it before the next `paint` cycle
      window.requestAnimationFrame(() => {
        let children: Record<string, Array<ChildNode>> = { '___': [] }
          Array.from(this.childNodes).forEach(elem => {
            let slotName = "___"
            if ((elem instanceof HTMLElement) && elem.getAttribute("slot")) slotName = elem.getAttribute("slot")
            if (!children[slotName]) children[slotName] = []
            children[slotName].push(elem)
            elem.remove()
          })
        let props: Record<string, any> = {
          slot: children,
          parent: this,
        }
    
        let attribs:Array<string>
        if (typeof(this.getAttributeNames)=="undefined"){
          attribs = Array.from(this.attributes).map(x=>x.name)
        } else {
          attribs = this.getAttributeNames()
        }
        attribs.forEach(attr => {
          props[attr] = this.getAttribute(attr)
        })
        let comp = this.svelteComp = new svelteElem({
          target: this,
          props
        })
        // transfer the properties and methods exposed by the component
        Object.getOwnPropertyNames(Object.getPrototypeOf(comp)).forEach(prop => {
          if (prop != "constructor") (this as any)[prop] = comp[prop]
        })
      })
    }

    // connectedCallback() {
    // }
    // disconnectedCallback(){
    // }
  }
  customElements.define(elemName, newElem);
}

To use it, you simply do this:

import MyComponent from './components/MyComponent.svelte'
elementify(MyComponent,"my-component-name")

After that you can use the <my-component-name> tag and the Svelte component will be mounted inside that component.

My solution might have some flaws but it's been working for months in production.